This invention relates to a security element for electronic article surveillance.
German published patent application DE 197 08 180 A1, for example, discloses a suitable security element comprising one lower conducting track and one upper conducting track, wherein the conducting tracks have a contact zone and, connected thereto, respective coil turns (windings) with opposite winding directions. The conducting tracks are superimposed, and an electrical connection exists between the lower conducting track and the upper conducting track in the area of the contact zone. The entire security element has at least one coil turn with an included angle of rotation of 2π. Security elements of this type are referred to as resonant circuits or radio frequency (RF) security elements, the resonant frequency being determined by the capacitance C, the inductance L and the resistance R of the resonant circuit.
Radio frequency security elements in the form of labels or tags are used in the prevention and detection of theft in department stores and warehouses, and they are becoming increasingly popular as the result of their high rates of detection.
Article surveillance itself can be described briefly as follows. In the interrogation zone of the area to be maintained under surveillance—this is conventionally the entrance and exit area of a department store or warehouse—the radio frequency security elements are excited by an alternating magnetic field into emitting a characteristic recognition signal. Once this characteristic signal is detected by the surveillance system, an alarm is produced. Visible to the visitor of a department store or warehouse are two columns of the surveillance system, through which each person wishing to leave the department store or warehouse must pass.
Essential for the detection rate of the surveillance system are the spatial distances between the above-mentioned columns, the level of existing disturbances and the performance of the RF security element.
One possibility for increasing the detection rate is to increase the dimensions of the security element. The disadvantage of this solution is the increased cost for the security element and, in addition, the difficulty encountered in equipping merchandise of small outer dimensions with such large security elements. Another disadvantage of the known RF security elements is the risk of the resonant frequency of the security elements being detuned, i.e., altered, by the presence of an electrically conducting mass in the vicinity of the security element. A knowledgeable shoplifter may utilize this effect by taking the security element in his hand, thereby reducing the detection rate drastically. Furthermore, it proves difficult to detect security elements of the type described with sufficient reliability when they are attached to products with a high water content. Such high water content products include, for example, meat, fish or drink bottles.